During times when students transfer, or classrooms change, the classrooms list can be reorganized by archiving accounts and classrooms that are no longer in use. 

Archiving a student or classroom means that all the data, progress and achievements remain, but students will not be able to access any of the challenges or courses available in the subscription.

Archiving will not affect the number of available seats, meaning it will not free-up seats in ‘Available Seats’ (unless you have a Flex subscription).
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​How to Remove (archive) a Classroom?